J2EE Patterns

When I am doing mock exams, my lowest score seams to be on J2EEpatterns. I have no idea why. I get 20% in J2EE Patterns and if you ask me, I am sure I grasped all the concepts. In head-first, it says that J2EE patterns should be a no-brainer. How do you suggest I prepare for J2EE patterns?